An optical fiber coupler couples light into and out of an optical fiber. However, coupling light into and out of an optical fiber presents a number of problems. In the case of multimode fibers, a lens is positioned and configured to focus the light of a well-collimated beam to a spot size that is less than the diameter of the fiber core. In addition, the angle from the lens to the fiber is less than the numerical aperture (“NA”) of the fiber core. In others words, the position and configuration of the lens used to inject light into the core of an optical fiber are selected to ensure that the maximal ray of the focused beam lies within the NA of the fiber core. On the other hand, in order to couple light into a single mode fiber, the mode of the fiber core and the mode of the light is nearly phase matched.
The lenses typically used to couple light into and out of either single or multimode fibers require a relatively large optical fiber coupler to house and position the lens at a fixed distance from the end of the fiber. In addition, the lens is typically the most expensive component of the optical fiber coupler.